Water Chemistry, Carryover, Steam Purity

The following discussions on water chemistry and steam purity apply to steam generators as well as waste heat boilers.

Good water chemistry is important for minimizing corrosion and the formation of scale in boilers. Steam-side cleanliness should be maintained in water tube as well as fire tube boilers. Plant engineers should do the following on a regular basis:

1. Maintain proper boiler water chemistry in the drum according to ABMA or ASME guidelines by using proper continuous blowdown rates. Chapter 6 discusses blowdown calculations and also gives the guidelines for feed and boiler water.

2. Ensure that the feed water analysis is fine and that there are no sudden changes in its conductivity or solids content.

3. Check the steam temperature to ensure that there are no sudden changes in its

value. A sudden change may indicate carryover of water vapor into steam. The author is aware of a project where the TOC limit in the feed water as recom- mended by ASME was well exceeded by the plant, and plant engineers were not aware of this. Suddenly, there was lot of foaming in the steam drum and carryover of water into the superheater, which decreased the steam temperature. The steam temperature was fluctuating as the organic compounds were causing varying degrees of foaming and the amount of water carried along with steam was varying. The superheater design and drum sizing were reviewed and found reasonable. The boiler load was also steady, and hence, plant engineers were won- dering what the problem was. An investigation was carried out on the feed water chemistry, and organic compounds were ascertained to be the cause. Fortunately, the problem was identified in a short period; the tube-side deposits were found to

be minimal, and hence, the superheater tubes did not fail.

4. Check the steam purity from time to time if the steam is used in a steam or gas

turbine (for NO x control). Monitoring the superheater tube wall temperatures is also good practice to ensure scales have not formed inside the tubes. Chapter 3 shows an example of the performance of a fire tube boiler with and without scale formation. Similar calculations on tube wall temperature and duty can be made in water tube steam generators also.

In the process of evaporating water to form steam, scale and sludge deposits form on the heated surfaces of a boiler tube. The chemical substances in the water concentrate in a film at the evaporation surface; the water displacing the bubbles of steam readily dissolves the soluble solids at the point of evaporation. Insoluble substances settle on the tube surfaces, forming a scale and leading to an increase in tube wall temperatures. Calcium bicarbonate, for example, decomposes in boiler water to form calcium carbonate, carbon dioxide, and water. Calcium carbonate has a limited solubility and will agglomerate at the heated surface to form

a scale. Blowdown helps remove some of the deposits. Calcium sulfate is more soluble than calcium carbonate and will deposit as a heat-deterrent scale. Most scale-forming substances have a decreasing solubility in water with an increase in temperature. Substances that have

a decreasing solubility with decrease in temperature are likely to deposit in steam turbine. In drum units, some moisture always entrains with the steam as it leaves the water

surface in the drum. Thus, drums are equipped with internal steam separators to remove

156 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

most entrained moisture and return it to the boiler water. However, complete removal of droplets is impossible, and thus trace amounts of solids will carry over to the superheater and main steam. These impurities include chlorides, sulfates, silica, phosphates (in phos- phate-treated units), and other compounds. A number of factors can cause carryover to be excessive. These include poor drum design, failed steam separators, improper drainage of separators, high dissolved solids in the boiler water, and excessive ramp rates during startup, among others.

Carryover can also be vaporous; certain compounds will volatilize and transfer the contaminants to steam. Silica and copper (in units with copper–alloy feed water heater tubes) are two notorious agents for vaporous carryover. Vaporous carryover is a function of steam density (pressure) and can be controlled only by limiting the various salts or silica, while mechanical carryover is controlled by using good steam separators and proper velocities in the steam space as discussed in Chapter 6.

Both mechanical and vaporous carryover become more pronounced with increasing pressure; thus, boiler water chemistry guidelines are increasingly stringent at higher pres- sures. The impurities affect turbine operation. The following list outlines a few of the important issues.

• Silica solubility decreases as steam pressure decreases through the turbine. Thus,

silica will deposit on turbine blades, particularly in the HP section. • Chloride and sulfate salts will deposit, commonly in the LP turbine. These

impurities, and chloride in particular, may cause pitting and stress corrosion cracking (SCC) of turbine blades and rotors. The most susceptible locations are the last stages, L-1 and L-0, where early condensate forms. During times of low load operation, early condensate formation may move backward a bit in the LP turbine.

• Sodium hydroxide (NaOH) can also cause SCC of turbine blades. • In units with copper–alloy feed water heaters that operate at or above 2,400 psig,

copper carryover can be quite detrimental. For the most part, copper precipitates in the HP turbine, where even a few pounds of deposition will reduce capacity by perhaps 10 MW or more. This capacity loss can be critical during periods of high power demand.

Steam purity requirements for saturated steam turbines are not stringent. As the steam begins to condense on the first stage of steam turbine water-soluble contaminants carried with the steam do not form deposits. However, there can be erosion concerns due to water droplets moving at high speeds.

If steam is superheated for use in steam turbines, steam purity is critical. Salts soluble in superheated steam may condense or precipitate and adhere to the metal surfaces as the steam is cooled when it expands. Deposition from steam can cause turbine valves to stick. Reduced efficiency and turbine imbalance and lower power output are the other concerns. Deposition and corrosion occur in the salt zone just above the saturation line and on the surfaces in wet steam zone. The solubility of all low-volatile impurities such as salts, hydroxides, silicon dioxide, and metal oxides decreases as steam expands in the turbine and is lowest at the saturation line. The moisture formed has the ability to dis- solve most of the slats and carry them downstream. The critical region for deposition of impurities in turbines operating on superheated steam is the blade row just upward of the Wilson line.

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When a desuperheater is used, the water used for spray should have the same purity as the final steam; else, the solids content of the steam will exceed the desired limits. The other option is to condense the steam and use the condensate for spray as discussed earlier.

Water Chemistry, Carryover, Steam Purity